20,402 research outputs found
A Machian Model of Dark Energy
Einstein believed that Mach's principle should play a major role in finding a
meaningful spacetime geometry, though it was discovered later that his field
equations gave some solutions which were not Machian. It is shown, in this
essay, that the kinematical models, which are invoked to solve the
cosmological constant problem, are in fact consistent with Mach's ideas. One
particular model in this category is described which results from the
microstructure of spacetime and seems to explain the current observations
successfully and also has some benefits over the conventional models. This
forces one to think whether the Mach's ideas and the cosmological constant are
interrelated in some way.Comment: Received an Honorable mention in the Essay Contest-2002 sponsored by
the Gravity Research Foundation; A paragraph added on how the model can
explain the CMB anisotropy observations; To appear in the Classical and
Quantum Gravit
ERTS and aircraft multispectral scanner digital data users manual
There are no author-identified significant results in this report
Origin of Radio Emission from Nearby Low-Luminosity Active Galactic Nuclei
We use the observational data in radio, optical and X-ray wavebands, for a
sample of active galactic nuclei (AGNs) with measured black hole masses, to
explore the origin of radio emission from nearby low-luminosity active galactic
nuclei (LLAGNs). We find that the radio luminosities are higher than the
maximal luminosities expected from the ADAF model, for most sources in this
sample. This implies that the radio emission is dominantly from the jets in
these sources. The radio emission from a small fraction of the sources in this
sample can be explained by the ADAF model. However, comparing the observed
multi-band emission data with the spectra calculated for the ADAF or ADIOS
cases, we find that neither ADAF nor ADIOS models can reproduce the observed
multi-band emission simultaneously, with reasonable magnetic field strengths,
for these radio-weak sources. A variety of other possibilities are discussed,
and we suggest that the radio emission is probably dominated by jet emission
even in these radio-weak LLAGNs.Comment: 25 pages, some references were added, accepted for publication in Ap
Chain Galaxies are Edge-On Low Surface Brightness Galaxies
Deep HST WFPC2 images have revealed a population of very narrow blue galaxies
which Cowie et al. (1996) have interpreted as being a new morphological class
of intrinsically linear star forming galaxies at . We show that the
same population exists in large numbers at low redshifts (z=0.03) and are
actually the edge-on manifestation of low surface brightness disk galaxies.Comment: 18 pages + 3 pages of figures. Uuencoded, gzipped, tar file of 1
latex file, 5 figures, and 2 latex style files. To appear in the
Astrophysical Journal Letter
A Gauge-invariant Analysis of Magnetic Fields in General Relativistic Cosmology
We provide a fully general-relativistic treatment of cosmological
perturbations in a universe permeated by a large-scale primordial magnetic
field, using the Ellis-Bruni gauge-invariant formalism. The exact non-linear
equations for general relativistic magnetohydrodynamic evolution are derived. A
number of applications are made: the behaviour of small perturbations to
Friedmann universes are studied; a comparison is made with earlier Newtonian
treatments of cosmological perturbations and some effects of inflationary
expansion are examined.Comment: 31 pages, Latex, Submitted to Classical and Quantum Gravit
Suzaku observations of Markarian 335: evidence for a distributed reflector
We report on a 151 ks net exposure Suzaku observation of the Narrow Line
Seyfert 1 galaxy Mrk 335. The 0.5-40 keV spectrum contains a broad Fe line, a
strong soft excess below about 2 keV and a Compton hump around 20-30 keV. We
find that a model consisting of a power law and two reflectors provides the
best fit to the time-averaged spectrum. In this model, an ionized, heavily
blurred, inner reflector produces most of the soft excess, while an almost
neutral outer reflector (outside ~40 r_g) produces most of the Fe line
emission. The spectral variability of the observation is characterised by
spectral hardening at very low count rates. In terms of our power-law +
two-reflector model it seems like this hardening is mainly caused by pivoting
of the power law. The rms spectrum of the entire observation has the curved
shape commonly observed in AGN, although the shape is significantly flatter
when an interval which does not contain any deep dip in the lightcurve is
considered. We also examine a previous 133 ks XMM-Newton observation of Mrk
335. We find that the XMM-Newton spectrum can be fitted with a similar
two-reflector model as the Suzaku data and we confirm that the rms spectrum of
the observation is flat. The flat rms spectra, as well as the high-energy data
from the Suzaku PIN detector, disfavour an absorption origin for the soft
excess in Mrk 335.Comment: 13 pages, 13 figures. Accepted for publication in MNRA
The Pooltable Analogy to Axion Physics
An imaginary character named TSP finds himself in a playroom whose floor is
tilted to one side. However, the pooltable in the playroom is horizontal. TSP
wonders how this can be. In doing so, he embarks upon an intellectual journey
which parallels that which has been travelled during the past two decades by
physicists interested in the Strong Problem and axion physics.Comment: 8 pages, plain TEX, 4 figures in a separate uuencoded postcript file;
based upon a talk at the XXXth Rencontres de Moriond 'Dark Matter in
Cosmology' and 'Clocks and Tests of Fundamental Laws', Villars-sur-Ollon,
Switzerland, Sept. 21-28, 199
Development of the electroweak phase transition and baryogenesis
We investigate the evolution of the electroweak phase transition, using a
one-Higgs effective potential that can be regarded as an approximation for the
Minimal Supersymmetric Standard Model. The phase transition occurs in a small
interval around a temperature T_t below the critical one. We calculate this
temperature as a function of the parameters of the potential and of a damping
coefficient related to the viscosity of the plasma. The parameters that are
relevant for baryogenesis, such as the velocity and thickness of the walls of
bubbles and the value of the Higgs field inside them, change significantly in
the range of temperatures where the first-order phase transition can occur.
However, we find that in the likely interval for T_t there is no significant
variation of these parameters. Furthermore, the temperature T_t is in general
not far below the temperature at which bubbles begin to nucleate.Comment: 26 pages, 7 figures; typos corrected, reference adde
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